1. Field of the Invention
The present invention relates to a process for converting polysaccharides, in particular lignocellulosic biomass material, in an inorganic molten salt hydrate to platform chemicals.
2. Description of the Related Art
In view of environmental concerns, there is a need for platform chemicals from renewable resources. The term platform chemicals is used to describe chemicals that are versatile starting materials for making specialty chemicals, and include sorbitol (or glucitol, alcohol sugar of glucose), xylitol/arabinitol (sugar alcohols from xylose and arabinose) and isosorbide (dianhydro-D-glucitol) and anhydrosugars [1].
It is considered a technical barrier to obtain (di) anhydro sugars production by selective dehydration of polyols, without side reactions. To produce such polyol dehydration products it is necessary to produce sugars, and subsequently hydrogenate such sugars to polyols. Several ways for producing sugars from lignocellulosic material are known in the art, and several ways for producing polyols from sugar are known in the art. A known method for producing sugars from cellulosic material is by acid hydrolysis.
U.S. Pat. No. 647,805 and U.S. Pat. No. 607,091 describe such hydrolysis processes, the first being a concentrated acid hydrolysis and the second a diluted acid hydrolysis. On the one hand, the diluted acid hydrolysis processes have a low yield, but do not need much further processing (acid removal) to separate and use the glucose formed. On the other hand, concentrated acid processes have higher yields but present difficulties in sugar recovery/acid separation. Processes for acid neutralization and removal, concentration of syrup and precipitation of sugars are known to those skilled in the art.
The fact that certain compounds are capable of dissolving cellulose are used in the art to derivatize cellulose to other chemicals. Heinze and coworkers [1], [2] provide an overview of the technology of dissolution of cellulose for derivatization.
Polysaccharides, such as cellulose, lignin and starch are easily dissolved in certain concentrated metal halides, like zinc halides ([3] and U.S. Pat. No. 257,607. Similarly, processes were developed to provide a faster, higher yield for cellulose hydrolysis to glucose, based on the concept of dissolution of the cellulose and further hydrolysis of cellulose to glucose in homogeneous media.
Calcium chloride concentrated solutions (5 to 55 wt %) with small amount of HCl (from 0.01% to 2 wt %) were used to hydrolyze cellulose to monosaccharides, U.S. Pat. No. 4,018,620. The calcium chloride was partially separated from the remaining solution by crystallization, but further removal of ions Ca2+ and Cl− were necessary. The swelling effect of the salt is believed to enhance the hydrolysis.
U.S. Pat. No. 4,452,640 discloses a process to dissolve and quantitatively hydrolyze cellulose to glucose without formation of degradation products, using ZnCl2 solutions. Dissolution was effected with salt solutions, with 60 to 80 wt % ZnCl2 being preferred, at sufficiently large contact time and temperatures of 70 to 180° C., preferably 100 to 145° C. After dissolution, it was claimed that lowering the ZnCl2 concentration (to 40 to 50 wt %) was further necessary prior to hydrolysis, to avoid glucose degradation, and subsequently HCl or a similar dilute acid was added to effect hydrolysis (down to pH<2).
A later publication of the same group showed results without the salt concentration lowering step [4]: experiments with dissolution media comprising 67 wt % of ZnCl2 were performed at temperatures of 50 to 100° C. and 2 hours time. Additional acid showed to be necessary to effect hydrolysis, 0.5 mol/L of solution being the optimum, with low conversion at lower concentrations and low yield at higher concentrations.
A reasonable temperature was 70° C., hydrolysis being incomplete at lower temperatures, and further conversion of glucose to other products at 100° C. Experimental ratios of ZnCl2 to cellulose were from 1.5 to 18. The higher the ZnCl2/cellulose ratio the higher was the yield of glucose. Contrary to previous teaching (U.S. Pat. No. 4,452,640), the presence of ZnCl2 lowered the degradation of glucose, in comparison to an aqueous solution of the same HCl content. Concentrated salt solutions were preferred, as solutions with increased water content were unable to dissolve cellulose, thus affecting the hydrolysis rate.
European Patent Application EP 0 091 221 A teaches the hydrolysis of cellulose or starch in solubilization media comprising water, an inorganic acid and hydrated halide of aluminum, optionally containing and additional metal halide, with yields close to 100%. Longer hydrolysis times than the necessary lead to a lower yield of glucose due to degradation.
Ragg and Fields from Imperial Chemical Industries (ICI) teach a process for hydrolysis of lignocellulosic waste using metal halides and hydrochloric acid as catalysts [5].
Several salts where tested to effect hydrolysis, LiCl and CaCl2 solutions were considered the most effective, ZnCl2, MgCl2 and CaCl2 being somewhat less effective, and FeCl3, SnCl4, NaCl, KCl, MnCl2, NiCl2, CuCl2, CoCl2 and CdCl2 being the least effective. Typical conditions were 60-90° C., 15 to 25 wt % of cellulose substrate, in a dissolution medium containing 5-40 wt % of CaCl2 and 25-40 wt % of HCl, pressure of 4-7 bar to maintain a liquid phase, and reaction times of 10 to 20 minutes, yielding more than 85 wt % of glucose.
Another advantage of the salts is the breaking of the azeotrope formed by HCl and water in reaction media with more than 15 wt % of CaCl2—making it easier to separate the HCl from the solution, which can be done with a simple evaporator.
Glucose, the desired product of processes of cellulose hydrolysis, needs to be further separated from the concentrated salt media in such processes. Besides precipitation of part of the salt (CaCl2 concentrated solutions, U.S. Pat. No. 4,018,620), ion exchange and chromatographic methods (U.S. Pat. No. 4,452,640 and [4]) or even electrodialysis [5] were considered.
In all these separation procedures, the salt—the main compound of the solution—is to be removed, which increases the separation cost. Other usual separation procedures such as vaporization cannot be used as glucose degrades at higher temperatures. Extraction is not an option, as both the salt and glucose are soluble in water.
A desired product of glucose is sorbitol, a hydrogenation product of glucose. A further desired product of sorbitol is the dianhydro sorbitol, or isosorbide, which is a product of double dehydration of sorbitol.
Reviews of Fléche [6] and Stoss [7] present the uses, properties and chemistry of isosorbide, the disclosures of which are incorporated herein by reference.
Methods for producing isosorbide involve the dehydration of sorbitol (D-glucitol) in acidic solutions. Protonation due to the presence of acid occurs preferentially at the primary glucitol hydroxyl group. The first internal dehydration step leads to 1-4 anhydro-D-glucitol. The dehydration can also take place at the 3 and 6 positions, leading to the 3,6-anhydro isomer [8]. Further dehydration of both isomers leads to the 1,4-3,6 dianhydro-D-glucitol, or isosorbide. Another possible first dehydration occurs at the 1,5 and 2,5 positions. In these positions no second intramolecular dehydration is observed, yielding the monoanhydride derivative. Another complicating problem is the possibility of intramolecular elimination of water between two molecules, leading to higher molecular oligomeric or polymeric units.
The teachings of most prior art patents deal with attempts of increasing the selectivity by preventing the formation of polymeric units and working in conditions where 1,5 and 2,5 dehydration products are less favored.
Acidic catalysts mainly used in the dehydration of sorbitol are H2SO4, phosphoric acid, HCl and other acids such as p-toluene sulfonic, methanesulfonic acid. Solid catalysts can be used such as acidic ion exchange resins, zeolites, and sulfated zirconia.
According to the state of the art of dehydration using acidic catalysts, dehydration conditions should be as anhydrous as possible. To accomplish this, dehydration is effected under vacuum (WO 00/14081), or with a flux of inert gas to effect water removal (for instance, using nitrogen, as taught by U.S. Pat. Nos. 6,407,266 and 6,689,892). Temperature limits are 170° C. in the presence of acid—above that significant char and tarring are to be expected, as U.S. Pat. No. 6,831,181 teaches.
It is possible to separate the isosorbide from the reaction mass using vacuum, as it has a vapor pressure of 2 mm Hg at 140° C.-145° C. and the vapor pressure of anhydroglucitol is just 0.04 mmHg at the same temperature. Process schemes involving separation and reaction using acidic catalysts are known in literature. U.S. Pat. No. 6,831,181 teaches such a process.
Besides the 1,5 and 2,5 monoanhydrohexitols, the formation of oligomeric and polymeric anhydrides is a problem—so, process schemes have been suggested whereby water is added after the reaction, to precipitate the polymers (but not dimers or monoanhydrides). In such a continuous process, a purge is necessary for removal of the non-reactive 1,5 and 2,5 monanhydrohexitols. Such procedures of recycle, precipitation and purge are taught by U.S. Pat. Nos. 6,831,181 and 6,864,378. Also, to further inhibit the formation of 2,5 monoanhydrohexitols, it is taught by US Patent Application 20070173651 to perform the reaction in acidic media in 2 temperature steps, a first step lower than 120° C., and a second step higher than 120° C. Furthermore, according to US Application 20070173652, it would be interesting to remove water from the polyol before the first dehydration, and after the first dehydration, and preferably during the first dehydration.
When using solid catalysts, such as acidic resins, to effect the dehydration, deactivation of the catalyst is a further problem. US 20070173653 teaches periodic catalyst washing with certain protic or aprotic solvents to ensure a longer catalyst life.
The literature also teaches procedures to prevent the formation of degradation oligomers and polymers involving hydrogenation under dehydration conditions, as in U.S. Pat. No. 6,013,812 and US Patent Application 20070173654.
US Patent Application 20070173654 teaches the use of a hydrogenation catalyst during dehydration of a (preferably anhydrous) sugar alcohol, in the presence of an acidic catalyst. The hydrogenation catalyst contains a metal selected from Pd, Pt, Ni, Co, Ru, Re, Rh, Ir and Fe, and a support, which is preferably carbon, or alternatively zirconia, titania, niobia, silica or tin oxide. It is also possible to employ bifunctional catalysts, combining acidic and hydrogenation functions. The pressure is lower than 35 bar, preferably less than 20 bar, or even less than 10 bar, and preferred temperatures range from 110° C. to about 170° C. The same patent teaches the possibility of using hydrogen flow in the countercurrent mode, as a way of effecting further water removal.
U.S. Pat. No. 6,013,812 teaches the use of hydrogenation and acidic catalysts in a hydrogen atmosphere to effect dehydration of polyols. Without claiming a particular catalyst, the authors used Pd/C and Ru/C and additional acids in the examples. In the presence of a catalyst, less than 1 wt % polymers, but significant amounts of low molar weight polyols were formed, products of metal catalyzed hydrogenolysis. Without acidic catalysts there is insufficient conversion of D-sorbitol. Raney Cu, Co/Cu/Mn, Raney Ni and Cr—Ni were also tested in the absence of acid, and in spite of a high conversion (hydrogenolysis) the formation of isosorbide was lower than 2 wt %.
MONTASSIER et al [9], [10], [11] teach the use of Cu/C or bimetallic copper catalysts to effect the dehydration of D-glucitol and other polyols under hydrogenation conditions, without added acids. Apparently, ionic copper compounds formed during the reaction in the catalyst surface are significantly electrophilic and capable of interacting strongly with the polyol hydroxyl groups, weakening the C—O bond, and thus catalyzing the formation of the cyclic internal dehydration products. Bimetallic copper-based catalysts, such as Cu/Ru, are also active, as the presence of Ru enhances the polarity of Cu. Unfortunately, the stability of such catalysts is extremely low (hours), due to the leaching of copper compounds. Hydrogenolysis byproducts are also formed. Stability of the catalyst could be enhanced to some extent by addition of NaCl.
U.S. Pat. No. 4,313,884 teaches that metal ions with a charge-to-ionic-radius ratio from about 2.0 to about 3.2 catalyze the dehydration of hexitols, at a temperature from about 100 to about 300° C., preferably from 150 to 250° C. A hexose, such as D-glucose, may be converted directly to anhydrohexitols by the hydrogenation in the presence of a hydrogenation catalyst to which the appropriate metal salt has been added. The metal ion-to-polyol ratio is from about 0.01 to about 0.1. Claimed salts are the ones with ions selected from the group consisting of magnesium, manganese, iron, cobalt, nickel, copper, actinium, thorium, protactinium, uranium, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. In the examples the dehydration of D-glucitol is effected in a 33 wt % aqueous solution containing a metal chloride at a mole ratio of 0.05 to D-glucitol, together with a nickel hydrogenation catalyst, without additional acid, yielding hydrogenolysis products (1,2-propylene glycol, ethylene glycol, glycerin and mannitol) and monoanhydro and dianhydro-D-glucitol.
DUCLOS et al. [12] teach that heating D-glucitol in anhydrous pyridine chloride (ionic liquid) at 120 to 160° C. for several hours (above 4 h) lead to 1,4-anhydro-D-glucitol and, to a lesser extent, 1,4:3,6-dianhydro-D-glucitol. Surprisingly, the authors observed no 1, 5 or 2,5 anhydro-D-glucitols.
Prior art publications mostly consider the use of anhydrous D-glucitol as feedstock, or prefer the removal of water present prior to dehydration and also during dehydration—so there are several previous steps to prepare feedstock to dehydration and additional production of glucitol from glucose, and production of glucose from cellulose or starch or other suitable means.
In none of the publications product was formed without by-products, such as polymerization products, 2,5-anhydro-D-glucitols, hydrogenolysis products—or with full conversion to the desired isosorbide product.
So there is a need for a process that is able to produce isosorbide with enhanced yield and with reduced formation of by-products.
Also, there is a need for processes to convert cellulose to glucose and further derivatives, preferentially a platform chemical, with enhanced conversion, such as those obtained in cellulose hydrolysis in homogeneous media.
Unfortunately, the separation of glucose from the dissolution agents is difficult in the hydrolysis in homogeneous media.
An object of the present invention is to solve or mitigate the above problems